At present, microprocessors are providing on-chip clock frequencies on the order of 1 GHz. The International Technology Roadmap for Semiconductors (ITRS) forecasts on-chip clock rates of about 10 GHz in 2011. Performance in the GHz range can only be fully exploited if the off-chip interconnection technology provides appropriate bandwidth, a constraint that will become increasingly more severe during the next ten years. This means that the challenge of routing signals off-chip and into the system in the GHz frequency range is expected to exceed that of achieving on-chip performance at these frequencies. In addition, highly parallel, next-generation computational systems will require highly dense connection networks containing many long-distance connections. In such highly connected, highly parallel systems, the module-to-module and long distance chip-to-chip connections are responsible for the majority of the power dissipation, time delay and surface area. Thus, it has become critically important to minimize the area, power and time delay of the chip-to-chip and module-to-module interconnects while, at the same time, increasing density and bandwidth. The Semiconductor Industry Association (SIA) has recognized these challenges and identifies interconnects as the primary chip-related technology with the largest potential technology gaps.
By replacing electrical intramodule and module-to-module connections with optical communication links, the communication bottleneck can be relieved. In recent years considerable R&D effort has been devoted to developing optical chip-to-chip interconnects to reduce this microelectronics interconnection problem. It has now been demonstrated that optical interconnects have the potential to increase communication speed and reduce the volume, crosstalk and power dissipation of the connections.
Despite recent progress and the demonstrated potential of optical interconnects, this technology is still at an early stage and practical realization of its potential will require more efficient approaches and further improvements in performance. In particular, there is a need for reconfigurable interconnect technologies capable of higher density of connections while reducing power, area and cost.
The phenomenon of surface plasmon resonance (SPR) provides the basis for the proposed interconnect microarray. When a metal surface is illuminated by transverse magnetic (TM)-polarized light of the appropriate wavelength, angle of incidence and phase velocity, a resonance condition occurs and energy from the light is coupled into the electrons of the metal to excite what is called a surface plasmon. A surface plasmon is a propagating electron density oscillation in a metal at a metal/dielectric interface. The associated electromagnetic fields constitute a guided surface mode of the conductor-dielectric interface and are evanescent in each medium with intensity decaying exponentially away from the surface. Penetration into the dielectric depends on the wavelength of the incident light and is typically of the order of a few hundred nm. When the conductor is a high-conductivity metal such as silver or gold and with air as the dielectric, the enhancement of the field intensity at the plasmon interface over that of the incident beam is about two orders of magnitude. A large fraction of the plasmon field energy lies in the dielectric, where it is available for sensing or modulation purposes.
The resonance condition (at which incident light couples into the surface plasmon) is manifested by a large fall in reflectance of the incident beam, the energy of which is transferred to the surface plasmon. The SPR resonance condition depends on the wavelength of the incident light, the angle of incidence and the index of refraction at the metal/dielectric interface. Surface plasmons cannot be directly excited by illuminating an isolated planar metal surface. The reason is that the phase velocity of the surface plasmon is slower than that of light and therefore their wave vectors cannot be matched to achieve resonance. This problem is typically circumvented by using the so-called prism-coupled technique, or Kretschmann configuration. In this method, a gold film is vapor-deposited onto a high-index glass prism. Illumination of the gold layer is performed through the prism at an angle greater than the critical angle for total internal reflection. The high index of refraction of the prism reduces the phase velocity of light. Under these conditions the surface plasmon can be excited by the evanescent light wave at the prism/metal interface. FIG. 2 shows a typical prism-coupled SPR arrangement.
A more practical alternative to prism coupling is grating-coupling. This is implemented by coating a diffraction grating with a thin film of a high-conductivity metal (exemplary materials are silver or gold). A diffraction grating is a periodically modulated interface between two media of different optical properties. A light wave incident on a grating is diffracted into various orders. With proper selection of incidence angle, wavelength of the incident light and grating periodicity, a higher diffracted order can be phase matched to the surface plasmon wave, as illustrated in FIG. 3. Grating coupling permits direct illumination of the metal surface and obviates the need for the cumbersome prism.
Until recently, most SPR systems have been based on prism coupling. Light modulation by prism-coupled SPR has been used for the study of voltage-dependent alignment in liquid crystals and a prism-coupled surface plasmon spatial light modulator based on a liquid crystal has been reported. The lack of grating-coupled systems has been primarily due to the greater complexity of fabrication that was previously required to make gratings. However, recent technological advances have reversed this situation. Plastic optical gratings can now be mass-produced at very low cost using the same technology that is used to produce digital video disks (DVDs) or compact disks (CDs) and could be formed in spin cast layers. Recently, the pharmaceutical industry developed a microarray-based grating-coupled SPR biosensor system for proteomics applications employing such a diffraction grating. This system is capable of massively-parallel detection of hundreds to thousands of protein binding events and of monitoring binding kinetics in real time.
Due to factors such as cost, physical size and additional necessary mounting hardware, the prism coupler is unsuitable for mass-produced, compact optical interconnects. Grating coupling, on the other hand, has significant advantages for optical interconnect applications in terms of size, cost and configuration flexibility. In addition, it allows smaller angles of incidence and hence increased aperture, provides greater spatial uniformity and permits higher refractive index electro-optical (EO) materials to be used. In addition, it enables easier integration with circuit boards, modules and integrated circuits.